[PATCH] fs/buffer.c: disable per-CPU buffer_head cache for isolated CPUs

[Marcelo Tosatti <mtosatti@redhat.com>]

For certain types of applications (for example PLC software or
RAN processing), upon occurrence of an event, it is necessary to
complete a certain task in a maximum amount of time (deadline).

One way to express this requirement is with a pair of numbers,
deadline time and execution time, where:

        * deadline time: length of time between event and deadline.
        * execution time: length of time it takes for processing of event
                          to occur on a particular hardware platform
                          (uninterrupted).

The particular values depend on use-case. For the case
where the realtime application executes in a virtualized
guest, an IPI which must be serviced in the host will cause
the following sequence of events:

        1) VM-exit
        2) execution of IPI (and function call)
        3) VM-entry

Which causes an excess of 50us latency as observed by cyclictest
(this violates the latency requirement of vRAN application with 1ms TTI,
for example).

invalidate_bh_lrus calls an IPI on each CPU that has non empty
per-CPU cache:

        on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);

The performance when using the per-CPU LRU cache is as follows:

 42 ns per __find_get_block
 68 ns per __find_get_block_slow

Given that the main use cases for latency sensitive applications
do not involve block I/O (data necessary for program operation is 
locked in RAM), disable per-CPU buffer_head caches for isolated CPUs.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/fs/buffer.c b/fs/buffer.c
index a7fc561758b1..49e9160ce100 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -49,6 +49,7 @@
 #include <trace/events/block.h>
 #include <linux/fscrypt.h>
 #include <linux/fsverity.h>
+#include <linux/sched/isolation.h>
 
 #include "internal.h"
 
@@ -1289,7 +1290,7 @@ static void bh_lru_install(struct buffer_head *bh)
 	 * failing page migration.
 	 * Skip putting upcoming bh into bh_lru until migration is done.
 	 */
-	if (lru_cache_disabled()) {
+	if (lru_cache_disabled() || cpu_is_isolated(smp_processor_id())) {
 		bh_lru_unlock();
 		return;
 	}
@@ -1319,6 +1320,10 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
 
 	check_irqs_on();
 	bh_lru_lock();
+	if (cpu_is_isolated(smp_processor_id())) {
+		bh_lru_unlock();
+		return NULL;
+	}
 	for (i = 0; i < BH_LRU_SIZE; i++) {
 		struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);